Fingerprinting is one of the most widely used biometric for human identification. Identification is obtained by analyzing a given fingerprint image obtained by a fingerprint sensor for the relative locations and orientations of structural elements such as branching or ending of ridges and valleys known as minutia. These characteristics are obtained in the enrollment mode of a person's finger or multiple fingers. In the verification mode a second fingerprint is obtained and analyzed for similarity based on minutia or other previously defined fingerprint characteristics. This minutia is also referred to herein as a type of biometric marker.
The probability for false identification either a false acceptance or false rejection depends on the number of minutia identified in the fingerprint. The number of minutia increases with the fingertip area being scanned. However, for integration of fingerprint sensors into mobile devices for access control, such as cell phone a small area fingerprint sensor is very desirable.
Sonavation, Inc. of Palm Beach Gardens, Fla., USA manufactures biometric sensing devices having a ceramic Micro-Electro Mechanical System (MEMS) piezoelectric array that is made from a ceramic composite material. When this piezoelectric material is formed into a pillar 1/10th the diameter of a human hair, it has a unique set of properties that enable it to mechanically oscillate when an electric field is applied or create an electrical voltage when mechanically vibrated. The piezoelectric pillar is electrically vibrated at its natural ultrasonic resonant frequency. If a fingerprint ridge is directly above the pillar, much of the ultrasonic energy is absorbed by the skin and the signal impedance of the pillar is very high. If a valley is directly above the pillar, very little energy is absorbed and the impedance is very low. By arranging the pillars in a matrix of several thousand elements a two-dimensional image of a fingerprint can be created. An imaging ASIC electrically controls the pillar oscillation, imaging of the fingerprint and data management of the fingerprint information.
U.S. Pat. No. 7,141,918 describes an biometric sensing device having the above piezoelectric array operable for fingerprint imaging. It has been found as also described in this patent that the piezoelectric array can be operated in non-fingerprint imaging modes to obtain other biometric information, such as in an echo mode to provide imaging, such as bone, or a Doppler-shift mode to detect blood flow velocity and blood flow patterns. Although the sensor described in this patent is useful, it would be desirable to also operate the sensing device in a three-dimension ultrasound imaging mode to provide improved imaging of subcutaneous structures for use in biometric identification (or medical applications) that does not rely on echo mode imaging as described in U.S. Pat. No. 7,141,918.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.